Mechanical efficiency is defined as the useful work over the total energy expended. This measurement would be most useful to quantitate the performance of the heart, particularly in the diagnosis and prognosis of human cardiovascular disease. The present research is preparatory to but excluding human application. Efficiency will be measured in intact dogs as work/work plus heat. The heat dissipation rate is obtained from the temperature difference between aortic and coronary sinus blood together with coronary sinus blood flow. Because not all heat generated by the left ventricle is recovered in the coronary sinus, the fractional distribution of left ventricular heat dissipation is obtained by simultaneously injecting known amounts of dye and heat (called "excess heat") into the left coronary artery. The fraction of "excess heat" recovered in the coronary sinus is usually smaller than the dye fraction. The missing heat must have been dissipated by routes other than the coronary sinus. It is assumed that the fractional distribution, thus derived, also applies to the heat generated by the left ventricle. Assuming further that the coronary sinus blood flow, left ventricular heat dissipation and, together with external work, mechanical efficiency may be obtained.